I've used migrationwiz and it did the job. I got some funky errors when I was migrating from O365 to O365 but going further into the documentation solved that for me. But if you're worried you can purchase support instances if you don't want to wait for your standard support email request to be responded to. All in all I'd use it again.

No problem at all. You can just buy one license and migrate on user. You can even do a trial migration for a user which will migrate 10 items in each folder up to 5 MB for free. Just sign up for an account here and start configuring your project.